卷曲和管径对单壁碳纳米管π轨道取向的影响

由于卷曲效应,单壁碳纳米管中π轨道的取向不再和石墨一样与三个σ键垂直,也不沿管的径向,而是与径向发生一定小角度的倾斜.π轨道的取向与单壁碳纳米管的管径和曲率有密切的关系.本文从几何结构出发,计算了(n,0),(n,n)和(n,m)三种单壁碳纳米管π轨道的倾角.结果表明三种倾角是不完全相同的.     

Wn (n= 1–6)团簇吸附CO的密度泛函研究

采用密度泛函理论(density functional theory, DFT) 在B3LYP/LANL2DZ基组水平上对钨团簇吸附CO分子进行了系统研究. 结果表明, W_nCO团簇的基态结构是在W_n团簇中性或阴离子基态结构的基础上吸附CO生长而成; CO的吸附以端位吸附为主,桥位吸附为辅; CO分子在W_n团簇表面发生的是非解离性吸附. 与优化的CO键长(0.116 nm)相比,吸附后C-O键长变长(0.120–0.123 nm), 表明吸附后C-O键被削弱, CO分子被活化了.稳定性分析表明,在所研究的团簇中, W₃CO和W₅CO团簇的稳定性较强;自然键轨道(NBO)分析表明, W原子与CO分子相互作用的本质是CO分子内的杂化轨道与W原子6s, 5d, 6p和6d轨道相互作用的结果. 关键词： nCO (n= 1–6)团簇')" href="#">W_nCO (n= 1–6)团簇 基态构型 稳定性 电子性质     

WmCn(m+n≤7团簇电子结构与光谱性质

采用密度泛函理论(DFT)中的杂化密度泛函B3LYP方法,在LANL2DZ基组水平上优化得到了Wm Cn(m+n≤7)团簇基态结构,并对其振动光谱、自然键轨道(NBO)进行了系统的研究.结果表明:在W原子和C原子作用的过程中,W原子的6s轨道上的电荷转移到了C原子2p轨道,产生了轨道杂化现象,形成了较强的W-C化学键;振动光谱分析显示振动频率主要分布在55.99cm-1至2061.41cm-1处,最强峰对应的的振动模式大部分都为C原子或W原子的伸缩振动.     

自旋轨道耦合作用对碳纳米管电子能带结构的影响

本文考虑自旋轨道耦合作用的情况下,采用紧束缚近似螺旋对称模型计算了单壁碳纳米管的电子能带结构.研究发现:对于Armchair型单壁碳纳米管,自旋轨道耦合作用和弯曲效应共同导致了费米面Dirac点附近电子能带结构的能隙;对于Zigzag型和手性单壁碳纳米管,自旋轨道耦合作用使得电子最高占据态和最低未占据态产生能级劈裂,能级劈裂的大小不但与碳纳米管的直径和手性角密切相关,而且相对于费米面是不对称的;根据指数(n,m)可以将Zigzag型和手性单壁碳纳米管分为金属性碳纳米管(ν=0) 关键词： 单壁碳纳米管 自旋轨道耦合 紧束缚近似螺旋对称模型     

WmCn(m+n≤7)团簇电子结构与光谱性质

采用密度泛函理论(DFT)中的杂化密度泛函B3LYP方法,在LANL2DZ基组水平上优化得到了WmCn(m+n≤7)团簇基态结构,并对其振动光谱、自然键轨道（NBO）进行了系统的研究.结果表明：在W原子和C原子作用的过程中,W原子的6s轨道上的电荷转移到了C原子2p轨道,产生了轨道杂化现象,形成了较强的W-C化学键; 振动光谱分析显示: 振动频率主要分布在55.99 cm-1至2061.41cm-1处,最强峰对应的的振动模式大部分都为C原子或W原子的伸缩振动.     

NiMgn(n=1-12)团簇的第一性原理研究

采用基于密度泛函理论(DFT)中的广义梯度近似(GGA),在考虑自旋多重度的情况下,对NiMgn(n=1-12)团簇进行了构型优化,频率分析和电子性质计算.结果表明:n=1,2时,体系的基态为自旋三重态,n≥3时.为单重态;Ni原子掺杂使主团簇结构发生了明显变化.n≤8时,三角双锥,四角双锥结构主导着NiMgn基态团簇的生长行为;n在9-12之间时,主团簇Mgn 1(n=1-12)的基于三棱柱构型的基态演化行为发生了一定程度的改变;n≥6时,Ni原子陷入了主团簇内部;掺杂使体系的平均结介能增大,能隙减小;n=4,6,10是团簇的幻数;不同尺寸团簇的s,p,d轨道杂化中,Ni原子3d,4p成分所起作用不同;NiMg6基态结构具有很高的对称性(Oh),很好的稳定性和化学活性,能隙仪为0.25 eV.     

(OsnN)0,±(n=1-6)团簇电子结构与光谱性质的理论研究

采用密度泛函理论中的杂化密度泛函B3LYP方法在赝势基组LANL2DZ水平上对Os_nN^0,±(n=1-6)团簇的各种可能构型进行了几何结构优化,得出了它们的基态构型,并对基态结构的磁学性质、自然键轨道(NBO)、光谱和芳香性进行了理论研究. 研究结果表明:OsN^-和Os₅N^-团簇发生了"磁矩猝灭"的现象,在Os₂N和Os₄N< 关键词： nN^0,±(n=1-6)团簇')" href="#">Os_nN^0,±(n=1-6)团簇 电子结构 光谱性质 密度泛函理论     

(n,n)-(2n, 0)碳纳米管异质结的扭转力学特性

相近直径的锯齿型和扶手椅型碳纳米管可以共轴组合形成5-7碳环交替出现的柱形对称异质结. 本文利用分子动力学方法研究了直径相近且等长锯齿型和扶手椅型碳纳米管形成的(n, n)-(2n, 0)结在扭转过程中的扭矩和轴向应力随扭转角度的变化规律以及应力传递过程. 研究发现, (n, n)-(2n, 0)结扭转应变在达弹性限度内不会产生轴向应力, 该效应对基于碳纳米管扭转特性的纳米振荡器件的设计具有重要意义.     

Rh_nAl(n=1～6)团簇结构和磁性的研究

用密度泛函理论中的广义梯度近似方法研究了Rh_nAl(n=1～6)团簇的结构和磁性.结果表明:Rh(_n-1)Al和Rh_n(n=2～7)团簇结构是相似的,结合能随团簇尺寸变化趋势一致,原子间的s,p,d轨道杂化使得Rh_nAl团簇更加稳定.几乎所有Rh原子都是电子受体,Al-Rh键长越小.Rh原子得电子就越多.团簇磁矩主要来自Rh原子的贡献,Rh原子的4d轨道磁矩是Rh原子磁矩的主要部分.Al原子失去的电子越多,则其磁矩就相对越小.     

Ru(0001)表面氮分子和钡原子的相互作用

本文采用密度泛函理论方法研究了Ru(0001)表面氮分子和钡原子的相互作用.计算结果表明,钡原子的作用弱化了氮分子键.氮分子键长从Ru(001)-N2表面的0.113 nm伸长互Ru(001)-N2/Ba表面的0.120 nm;分子的拉伸振动频率从2221 cm-1减小到1746 cm-1;氮分子得到的电荷数从清洁表面的0.3e增加到1.1 e.电荷从钡原子6s轨道向钌原子4d轨道转移,转移电荷增强了氮分子2π空轨道和钌原子4d轨道间的杂化作用,导致5σ分子轨道和dπ杂化轨道发生极化.轨道极化使分子电偶极矩增加了约-0.136 e(A).金属钡在Ru(0001)表面氮分子活化过程中具备电子型助催剂的特征.     

First-principles calculation of the electronic structure and energy loss near edge spectra of chiral carbon nanotubes

We present first principles calculations of the electronic structure of small carbon nanotubes with different chiral angles theta and different diameters (d<1 nm). Results are obtained with a full potential method based on the density functional theory (DFT), with the local density approximation (LDA). We compare the band structure and density of states (DOS) of chiral nanotubes with those of zigzag and armchair tubes with similar diameters. The carbon K-edge energy loss near edge structures (ELNES) have been studied and pi* and sigma* contributions have been evaluated. These contributions give information on the degree of hybridization for the small chiral nanotubes.     

The calculation of energy gaps in small single-walled carbon nanotubes within a symmetry-adapted tight-binding model

This paper studies in detail the electronic properties of the semimetallic single-walled carbon nanotubes by applying the symmetry-adapted tight-binding model.It is found that the hybridization of π-σ states caused by the curvature produces an energy gap at the vicinity of the Fermi level.Such effects are obvious for the small zigzag and chiral single-walled carbon nanotubes.The energy gaps decrease as the diameters and the chiral angles of the tubes increase,while the top of the valence band and the bottom of the conduction band of armchair tubes cross at the Fermi level.The numeral results agree well with the experimental results.     

Mechanical properties of single- and double-walled carbon nanotubes under hydrostatic pressure

Based on a molecular mechanics coupled with atomistic-based continuum theory, a closed-form formula is presented to examine the elastic properties of single- and double-walled carbon nanotubes subjected to hydrostatic pressure. Following the present model, the effects of the armchair and zigzag CNT structures on the pressure behavior are theoretically investigated. The computational result indicates that the bulk modulus is less sensitive to the chiral structures except for very small tube diameters. Moreover, closed-end nanotubes under hydrostatic pressure exhibit a larger bulking modulus than open ended nanotubes. The cap of the zigzag tubes has a larger effect on the bulk modulus when compared to the armchair tubes, especially in small diameter nanotubes. The predicted strain and the bulk modulus are in good agreement with existing theoretical results. PACS 61.46.+w; 62.20.Dc; 62.20.-x; 62.25.+g     

单壁碳纳米管能带及其电子特性研究

从能带理论出发,采用电子紧束缚能量色散关系,推导锯齿,扶手椅和手性单壁碳纳米管(SWCNT)的电子能带结构表达式,指出单壁碳纳米管或为金属或为半导体的判据。结果表示:单壁碳纳米管的电子结构与其几何结构密切相关,如扶手椅型单壁碳纳米管是金属性的,而对其它类型的单壁碳纳米管是与碳纳米管的手性指数有关,只有手性指数n和m的差别等于3的倍数时,单壁碳纳米管是金属性的,否则会显出有带隙的半导体特性。这意味着单壁碳纳米管是由特殊的电子传输和光学性质,在纳米电子学领域具有巨大的潜在应用价值。     

Thermal conductivity for single-walled carbon nanotubes from Einstein relation in molecular dynamics

Equilibrium molecular dynamics based Einstein relation with an appropriate definition for integrated heat current (i.e., with modified energy moment) are combined to quantify the thermal conductivity of individual single-walled carbon nanotubes, armchair, zigzag and chiral tubes. The thermal conductivity has been investigated as a function of three parameters, tube radius, length and chirality at and near room temperature with Brenner potential model. Thermal conductivity is found to have unusually high value and varies with radius, length and chirality of tubes. Also the thermal conductivity at temperature range from 50 to 100 K is found to have a maximum value. For 12.1 nm tube length, the thermal conductivity has converging trend which its value dependents on the tube radius and chirality. Tubes with large radius have lower values of thermal conductivity. Furthermore, the results show that armchair tubes have large values of the thermal conductivity comparing with zigzag and chiral tubes. It seems possible to uncover carbon nanotubes thermal properties based on measurements having heat dependence by adding another methods for calculations.     

First principles study of work functions of single wall carbon nanotubes

We perform first principles calculations on work functions of single wall carbon nanotubes, which can be divided into two classes according to tube diameter (D). For class I tubes (D > 1 nm), work functions lie within a narrow distribution (approximately 0.1 eV) and show no significant chirality or diameter dependence. For class II tubes (D < 1 nm), work functions show substantial changes, with armchair tubes decreasing monotonically with diameter, while zigzag tubes show the opposite trend. Surface dipoles and hybridization effects are shown to be responsible for the observed work function change.     

形变碳纳米管场效应晶体管的电学性质

在紧束缚理论的基础上,推导出轴向拉伸和扭转形变时碳纳米管（CNT）的能带公式.结果显示拉伸和扭转形变都可以改变CNT的导电性质,在金属型和半导体型之间转变,特别是对于锯齿型CNT,根据n 与3的余数关系,在拉伸和扭转中分别显示出三种不同的变化规律.进一步应用场效应晶体管Natori理论模拟计算形变对CNT场效应晶体管的电流-电压特性的影响,锯齿型CNT根据n 与3的余数关系表现出不同的电流变化趋势,而对于扶手椅型CNT轴向拉伸不改变电流;在扭转形变时,CNT电流急剧升高,特别是扶手椅型CNT.锯齿型CNT和扶手椅型CNT的电流随扭转角度和外电压行为明显不同.在某些特定的扭转角度,电流随扭转角度变化非常显著,显示出锯齿型CNT和扶手椅型CNT发生半导体型与金属型之间的转变. 关键词： 碳纳米管 紧束缚理论 费米能级 能带结构     

Effect of tubular chiralities of single-walled ZnO nanotubes on electronic transport

The electronic transport properties of single-walled ZnO nanotubes with different chiralities are investigated by nonequilibrium Green's function combined with density functional theory. In this paper we consider three representative ZnO nanotubes, namely (3, 3) armchair, (5, 0) zigzag, and (4, 2) chiral, with a similar diameter of about 5.4 Å. Short nanotubes exhibit good conductance behavior. As the tube length increases, the conductance decreases at low bias and the nanotubes indicate semiconducting behavior. The current-voltage characteristics of the nanotubes longer than 3 nm depend weakly on the length of the tubes. The armchair and chiral ZnO nanotubes with the same length and diameter have almost overlapped current-voltage curves. The electron transport behaviors are analyzed in terms of the transmission spectra, density of states and charge population of these nanotubes. The results indicate that the resonant peaks above the Fermi level are responsible for electric currents. However, the zigzag ZnO nanotubes exhibit asymmetric current-voltage curves attributed to the built-in polarization field and give larger current than the armchair and chiral nanotubes at the same bias. The features explored here strongly suggest that the ZnO nanotubes are stable, flexible structures, which are valuable in Nano-Electromechanical System.     

First-principles study of narrow single-walled GaN nanotubes

Structural and electronic properties of narrow single-walled GaN nanotubes with diameter from 0.30 to 0.55 nm are investigated using the density functional method with generalized-gradient approximation. The calculations of total energies predict that the most likely GaN nanotubes in our calculation are (2,2), (3,2) and (3,3) nanotubes. From a detailed analysis we find that these narrow single-walled GaN nanotubes are all semiconductors, of which the armchair and chiral tubes are indirect-band-gap semiconductors whereas the zigzag ones have a direct gap except for (4,0) tube. The indirect band gap of (4,0) tube can stem from band sequence change induced by curvature effect. Our results show that the π-π hybridization effect and the formation of benign buckling separations play a key role in the band sequence changes of (4,0) tube.     

Ab initio study of dielectric function of C-substituted single walled boron nanotubes

We report dielectric function related optical properties namely dielectric constant, static dielectric constant, and absorption coefficients of C-substituted hexagonal boron nanotubes. The optical properties were computed for parallel and perpendicular polarized light in the framework of density functional theory. In this regard, three models of BNTs namely armchair (3,3), zigzag (5,0), and chiral (4,2) have been undertaken for probing the effect of carbon impurity. Our calculations show high dielectric constant of armchair and chiral BNTs for parallel polarized light and magnitude becomes smaller for higher impurity concentration, while zigzag BNT exhibits reverse trend for high impurity concentration. For perpendicular polarized light, the magnitude of dielectric constant ε ₁(ω) is decreased and shifts at higher frequencies. The absorption is revealed highest for armchair followed by zigzag and chiral BNTs independent of impurity concentration. The intensity of absorption gets weaken for higher concentration. The chiral BNTs show smaller but uniform absorption in smaller frequency range results in uniform field emission. These findings are also compared with available experimental and theoretical results. These metallic nanotubes are promising candidate as interconnects for nanodevices as well as field emission devices.